FIGS. 19 and 20 show an example of an electric power steering device that is known as the prior art and disclosed in Patent Document 1 or the like. In the case of the electric power steering device, a steering shaft 2 that has a rear edge to which a steering wheel 1 is fixed is rotatably supported inside a cylindrical steering column 3 A movement of the steering wheel 1 during steering is transmitted to an input shaft 8 of a steering gear unit 7 via the steering shaft 2, an electric assist device 4, a universal joint 5a, an intermediate shaft 6, and another universal joint 5b. When the input shaft 8 rotates, a pair of tie rods 9 and 9 that are placed on both sides of the steering gear unit 7 are pushed and pulled, and a steering angle in accordance with an operation amount of the steering wheel 1 is applied to a pair of left and right steering wheels.
In the present specification, a longitudinal direction is a traveling direction of a car. In all the drawings except for FIGS. 2, 6, 11, and 16 to 18, the left side is a “front side” and the right side is a “rear side”.
As illustrated in FIG. 20, the electric assist device 4 is provided with an electric motor 10 that is an auxiliary power source, a metallic housing 11 that supports the electric motor 10, and an output shaft 12 as a torque transmission shaft, a torsion bar 13, a torque detector 14, and a worm-type speed reducer 15 that are placed inside the metallic housing 11.
The housing 11 is formed by a lid 16 on the front side and a main body 17 on the rear side being coupled with each other by a plurality of bolts 18 and 18 and is coupled with and fixed to a front edge of the steering column 3. A front edge of the steering shaft 2 is inserted into the housing 11.
The output shaft 12 is formed of steel and has a hollow shape, and is rotatably supported by a pair of ball bearings 19 and 20 on a front side of the steering shaft 2 in the housing 11. A front edge of the output shaft 12 that protrudes from a front end opening of the housing 11 is coupled with the universal joint 5a (refer to FIG. 19).
The torsion bar 13 is formed of steel such as spring steel, and is inserted into a inner diameter side of the output shaft 12. In this state, a rear edge of the torsion bar is press-fitted into a coupling hole portion 21 disposed on a inner diameter side of the steering shaft 2, and is connected to be capable of torque transmission to the steering shaft 2. A front edge of the torsion bar 13 is connected to be capable of torque transmission to the output shaft 12 by a connecting pin 22 in a state of being internally fitted into the front edge of the output shaft 12. Specifically, outer diameter side through-holes 23 and 23 are formed at two positions that are sides of the front edge of the output shaft 12 opposite to each other in the radial direction, and an inner diameter side through-hole 24 are formed at the front edge of the torsion bar 13. The through-holes 23 and 23 and the inner diameter side through-hole 2 4 are formed concentrically to each other and in a radial direction. The connecting pin 22 is press-fitted inside each of the outer diameter side and inner diameter side through-holes 23 and 24 in a state of being laid across between the respective outer diameter side and inner diameter side through-holes 23 and 24.
The torque detector 14 is provided with an uneven portion 25 for torque detection, a sleeve 26 for torque detection, and a coil unit 27 for torque detection. The uneven portion 25 for torque detection is disposed at a part close to a rear end of an outer peripheral surface of the output shaft 12. The sleeve 26 for torque detection is supported by and fixed to the front edge of the steering shaft 2 in a state of being placed around the uneven portion 25 for torque detection. The coil unit 27 for torque detection is supported by and fixed to the housing 11 in a state of being placed around the sleeve 26 for torque detection.
The worm-type speed reducer 15 is formed by a combination of a worm wheel 28 and a worm (not illustrated). The worm wheel 28 is externally fitted into and fixed to a substantially central portion in an axial direction that is a part of the output shaft 12 between the ball bearings 19 and 20. The worm is rotatably supported in the housing 11 in a state of meshing with the worm wheel 28. A base edge of the worm is coupled with an output shaft of the electric motor 10 to be capable of torque transmission.
In the case of the electric power steering device configured as described above, the torsion bar 13 is elastically twisted in accordance with the direction and magnitude of the torque that is a steering force which is applied from the steering wheel 1 to the steering shaft 2. As a result, a positional relationship of the uneven portion 25 for torque detection and the sleeve 26 for torque detection in a circumferential direction is changed, and thus a coil that constitutes the coil unit 27 for torque detection undergoes a change in impedance. Accordingly, the direction and magnitude of the torque can be detected based on this impedance change. The electric motor 10 generates auxiliary power in accordance with the direction and magnitude of the torque. This auxiliary power is applied to the output shaft 12 after being increased by the worm-type speed reducer 15. As a result, a force that is required for a driver's operation of the steering wheel 1 is reduced.
In the case of the electric power steering device described above, a base portion 31 of a yoke 29 is externally fitted into and fixed to the front edge of the output shaft 12 as illustrated in, for example, FIG. 21. The base portion 31 has a partially cut cylindrical shape, and the yoke 29 is one of a pair of yokes 29 and 30 that constitute the universal joint 5a. 
In the case of the external fitting and fixing described above, one edge of the connecting pin 22 might reach a position within a range of a discontinuous portion (slit) 33 that is formed in the base portion 31 of the yoke 29 as in the case of the structure which is illustrated in FIG. 21. In this case, the possibility of a displacement of the connecting pin 22 with respect to the output shaft 12a in the axial direction of the connecting pin 22 resulting from repeated use of the torque transmission unit cannot be ruled out provided that a width dimension of the discontinuous portion 33 exceeds a diameter dimension of the connecting pin 22. Accordingly, there is room for improvement.
In the case of the structure that is illustrated in FIG. 22, both edges of the connecting pin 22 remain hidden by an inner diameter side of the base portion 31. Accordingly, in a stage after the yoke 29 is coupled with and fixed to the front edge of the output shaft 12 during an inspection at an assembly line, for example, it cannot be visually confirmed (or an image diagnosis-based confirmation after camera-based imaging cannot be carried out) from the outside of the yoke 29 whether or not the connecting pin 22 is installed inside each of the outer diameter side and inner diameter side through-holes 23 and 24. In other words, the confirmation of whether or not the connecting pin 22 is installed has to be performed in a stage before the yoke 29 is coupled with and fixed to the front edge of the output shaft 12. Accordingly, there is room for improvement.